Stegosaurians were armored dinosaurs (thyreophorans) and originally did not differ much from more primitive members of that group, being small, low-slung, running animals protected by armored scutes. An early evolutionary innovation was the development of tail spikes, or "thagomizers", as defensive weapons. Later species, belonging to a subgroup called the Stegosauridae, became larger, and developed long hindlimbs which no longer allowed them to run. This increased the importance of active defence by the thagomizer, which could ward off even large predators because the tail was in a higher position, pointing horizontally to the rear from the broad pelvis. Stegosaurids had complex arrays of spikes and plates running along their backs, hips and tails. Their necks became longer and their small heads became narrow, able to selectively bite off the best parts of cycads with their beaks. When these plant types declined in diversity, so did the stegosaurians, which became extinct during the first half of the Cretaceous period.

The first stegosaurian finds in the early 19th century were fragmentary. Better fossil specimens, of the genus Dacentrurus, were discovered in 1874 in England. Soon after, in 1877, the first nearly-complete skeleton was discovered in the USA. Professor Othniel Charles Marsh that year classified those specimens in the new genus Stegosaurus, from which the group acquired its name, and which is still by far the most famous stegosaurian. During the latter half of the twentieth century, many important Chinese finds were made, representing about half of the presently known diversity of stegosaurians.

Stegosaurians had characteristic small, long, flat, narrow heads and a horn-covered beak or rhamphotheca,[1] which covered the front of the snout (two premaxillaries) and lower jaw (a single predentary) bones. Similar structures are seen in turtles and birds. Apart from Huayangosaurus, stegosaurs subsequently lost all premaxillary teeth within the upper beak. Huayangosaurus still had seven per side.[2] The upper and lower jaws are equipped with rows of small teeth. Later species have a vertical bone plate covering the outer side of the lower jaw teeth. The structure of the upper jaw, with a low ridge above, and running parallel to, the tooth row, indicates the presence of a fleshy cheek. In stegosaurians the typical archosaurian skull opening, the antorbital fenestra in front of the eye socket, is small, sometimes reduced to a narrow horizontal slit.

All stegosaurians are quadrupedal, with hoof-like toes on all four limbs. All stegosaurians after Huayangosaurus have forelimbs much shorter than their hindlimbs. Their hindlimbs are long and straight, designed to carry the weight of the animal while stepping. The condyles of the lower thighbone are short from the front to the rear. This would have limited the supported rotation of the knee joint, making running impossible. Huayangosaurus had a thighbone like a running animal. The upper leg was always longer than the lower leg.

Huayangosaurus had relatively long and slender arms. The forelimbs of later forms are very robust, with a massive humerus and ulna. The wrist bones were reinforced by a fusion into two blocks, an ulnar and a radial. The front feet of stegosaurs are commonly depicted in art and in museum displays with fingers splayed out and slanted downward. However, in this position most bones in the hand would be disarticulated. In reality, the hand bones of stegosaurs were arranged into vertical columns, with the main fingers, orientated outwards, forming a tube-like structure. This is similar to the hands of sauropod dinosaurs, and is also supported by evidence from stegosaurian footprints and fossils found in a lifelike pose.[3]

Stegosaurus mount showing to a good effect the high neck posture, the throat ossicles and the robust shoulder girdle and forelimbs

The long hindlimbs elevated the tail base and the tail from that high position pointed almost horizontally to behind. While walking, the tail would not have sloped downwards as this would have impeded the function of the tail base retractor muscles, to pull the thighbones backwards. However, it has been suggested by Robert Thomas Bakker that stegosaurs could rear on their hind legs to reach higher layers of plants, the tail then being used as a "third leg". The mobility of the tail was increased by a reduction or absence of ossified tendons, that with many Ornithischia stiffen the hip region. Huayangosaurus still possessed them. In species that had short forelimbs, the relatively short torso towards the front curved strongly downwards. The dorsal vertebrae typically were very high, with very tall neural arches and transverse processes pointing obliquely upwards to almost the level of the neural spine top. Stegosaurian back vertebrae can easily be identified by this unique configuration. The tall neural arches often house deep neural canals; enlarged canals in the sacral vertebrae have given rise to the incorrect notion of a "second brain". Despite the downwards curvature of the rump, the neck base was not very low and the head was held a considerable distance off the ground. The neck was flexible and moderately long. Huayangosaurus still had the probably original number of nine cervical vertebrae; Miragaia has an elongated neck with seventeen.[4]

The stegosaurian shoulder girdle was very robust. In Huayangosaurus on the shoulderblade the acromion, a process on the lower front edge, was moderately developed; the coracoid was about as wide as the lower end of the scapula with which it formed the shoulder joint. Later forms tend to have a strongly expanded acromion, while the coracoid, largely attached to the acromion, no longer extends to the rear lower corner of the scapula. Ossified sternal plates have never been found with Stegosauria and perhaps the sternum was completely absent.

The stegosaurian pelvis originally was moderately large, as shown by Huayangosaurus. Later species however, convergent to the Ankylosauria developed very broad pelves, in which the iliac bones formed wide horizontal plates with flaring front blades to allow for an enormous belly-gut. The ilia were attached to the sacral vertebrae via a sacral yoke formed by fused sacral ribs. Huayangosaurus still had rather long and obliquely oriented ischia and pubic bones. In more derived species these became more horizontal and shorter to the rear, while the front prepubic process lengthened.

Like all Thyreophora, stegosaurians were protected by bony scutes that were not part of the skeleton proper but skin ossifications instead: the so-called osteoderms. Huayangosaurus had several types. On its neck, back, and tail were two rows of paired small vertical plates and spikes. On the rear of the tail, pairs of spikes were present forming the so-called "thagomizer", a defensive weapon. The very tail end bore a small club. Each flank had a row of smaller osteoderms, culminating in a long shoulder spine in front, curving to the rear. Later forms show very variable configurations, combinating plates of various shape and size on the neck and front torso with spikes more to the rear of the animal. They seem to have lost the tail club and the flank rows are apparently absent also, with the exception of the shoulder spine, still shown by Kentrosaurus and extremely developed, as its name indicates, in Gigantspinosaurus. As far as is known, all forms possessed some sort of thagomizer, though these are rarely preserved articulated allowing to establish the exact arrangement. A fossil of Chungkingosaurus sp. has been reported with three pairs of spikes pointing outwards and a fourth pair pointing to the rear.[5] The most derived species, like Stegosaurus, Hesperosaurus and Wuerhosaurus, have very large and flat back plates. To discern them from the smaller plates, which are intermediate to spines in having a thickened central section, these latter are sometimes called 'splates'. Stegosaurus plates are so large that it has been suggested that they were not arranged in paired but alternated rows or even formed a single overlapping midline row. With Stegosaurus fossils also ossicles have been found in the throat region, bony skin discs that protected the lower neck. Apart from protection, suggested functions of the osteoderms include display, species recognition and thermoregulation.[6]

Stegosaurian tracks were first recognized in 1996 from a hindprint-only trackway discovered at the Cleveland-Lloyd quarry, which is located near Price, Utah.[7] Two years later, a new ichnogenus called Stegopodus was erected for another set of stegosaurian tracks which were found near Arches National Park, also in Utah.[7] Unlike the first, this trackway preserved traces of the forefeet. Fossil remains indicate that stegosaurs have five digits on the forefeet and three weight-bearing digits on the hind feet.[7] From this scientists were able to predict the appearance of stegosaurian tracks in 1990, six years in advance of the first actual discovery of Morrison stegosaurian tracks.[7] More trackways have been found since the erection of Stegopodus. None, however, have preserved traces of the front feet and stegosaurian traces remain rare.[7]

Huayangosaurus is the oldest and most basal stegosaurian of which good material is known, giving an impression of the build of the earliest members of the group

Like the spikes and shields of ankylosaurs the bony plates and spines of stegosaurs evolved from the low-keeled osteoderms characteristic of basal thyreophorans.[8] One such described genus, Scelidosaurus, is proposed to be morphologically close to the last common ancestor of the clade uniting stegosaurs and ankylosaurs, the Eurypoda.[9] The perhaps most basal known stegosaurian, the four metres long Huayangosaurus, is still close to Scelidosaurus in build, with a higher and shorter skull, a short neck, a low torso, long slender forelimbs, short hindlimbs, large condyles on the thighbone, a narrow pelvis, long ischial and pubic shafts and a relatively long tail. Its small tail club might be a eurypodan synapomorphy. Huayangosaurus lived during the Bathonian stage of the Middle Jurassic, about 166 million years ago.

From the Early Cretaceous far fewer finds are known and it seems that the group had declined in diversity. Some fragmentary fossils have been described such as Craterosaurus from England and Paranthodon from South-Africa. The only more substantial discoveries are those of Wuerhosaurus from the Aptian-Albian of China, about 110 million year old. This also represents the latest stegosaurian known; it might be that the Stegosauria have become extinct before the Late Cretaceous. Dravidosaurus from the Coniacian of India, in 1979 proposed as a late-surviving stegosaur, in 1991 proved to have been based on plesiosaurian pelvis and hindlimb material instead.[10] It has often been suggested that this decline was part of a Jurassic/Cretaceous transition, a faunal turnover caused by a floral turnover, the new group of the angiosperms becoming the dominant plants which in turn led to new groups of herbivores. Although in general the case for such a causal relation is poorly supported by the data, stegosaurians are an exception in that their decline coincides with that of the Cycadophyta.[11] Stegosaurians, with their small snouts, could have selectively fed on the high quality parts, such as the cones, of these plants. They might have been succeeded in their rôle of low-level browsers by the Ankylosauria, which possessed broad snouts to crop large amounts of low-quality fodder.

The Huayangosauridae were an early group of stegosaurs which lived during the early to middle Jurassic Period. They were smaller than later stegosaurs and had shorter and higher skulls. Huayangosauridae is undefined. Currently, the only unequivocal genus included is the type genusHuayangosaurus of China. The poorly known remains of Regnosaurus from the Cretaceous of England, however, indicate it too could be a member — or at least a basal stegosaurian. They consist of a lower jaw that is very similar to that of the former genus.

The vast majority of stegosaurian dinosaurs thus far recovered belong to the Stegosauridae, which lived in the later part of the Jurassic and early Cretaceous, and which were defined by Paul Sereno as all stegosaurs more closely related to Stegosaurus than to Huayangosaurus.[13] They include per definition the well-known Stegosaurus. This group is widespread, with members across the Northern Hemisphere, Africa and possibly South America.[14]

To date, several genera from China bearing names have been proposed but not formally described, including "Changdusaurus".[17] Until formal descriptions are published, these genera are regarded as nomina nuda. Yingshanosaurus, for a long time considered a nomen nudum, transpired to have been described in 1994.[18]

Kenneth Carpenter of the Denver Museum of Nature and Science published a preliminary phyletic tree[19] of stegosaurs, in the 2001 description of Hesperosaurus. Here, the basal stegosaurian Huayangosaurus is used as the outgroup. The Stegosauridae are then defined as all stegosaurs closer to Stegosaurus than to Huayangosaurus. The position of Chungkingosaurus is uncertain due to lack of data.

The first known discovery of a possible stegosaurian, was probably made in the early nineteenth century in England. It consisted of a lower jaw fragment and was in 1848 named Regnosaurus. In 1845, in the area of the present state of South-Africa remains were discovered that much later would be named Paranthodon. In 1874, from England Craterosaurus was named. All three taxa were based on fragmentary material and were not recognised as possible stegosaurs until the twentieth century. They gave no reason to suspect the existence of a new distinctive group of dinosaurs.

In 1874, extensive remains, the first partial stegosaurian skeleton known,[20] were uncovered in England of what was clearly a large herbivore equipped with spikes. They were named Omosaurus by Richard Owen in 1875. Later this name was shown to be preoccupied by the phytosaurOmosaurus and the stegosaurian was renamed Dacentrurus. Other English nineteenth century and early twentieth century finds would be assigned to Omosaurus; later they would, together with French fossils, be partly renamed Lexovisaurus and Loricatosaurus. None of these specimens was complete though, and even together they could not have provided a good understanding of stegosaurian build. Owen e.g., initially assumed that the spikes were placed on the wrists. However, very soon after the discovery of Omosaurus, American finds would fully compensate for this.

Stegosaurus bones illustrated by Arthur Lakes in 1879

In 1877, Arthur Lakes, a fossil hunter working for Professor Othniel Charles Marsh, in Wyoming excavated a fossil that Marsh the same year named Stegosaurus. At first Marsh still entertained some incorrect notions about its morphology. He assumed the plates formed a flat skin cover — hence the name, meaning "roof saurian" — and that the animal was bipedal with the spikes sticking out sideways from the rear of the skull. A succession of additional discoveries from the Como Bluff sites allowed a quick update of the presumed build. In 1882 Marsh was able to publish the first skeletal reconstruction of a stegosaur. Hereby, stegosaurs became much better known to the general public. The American finds at the time represented the bulk of known stegosaurian fossils, with about twenty skeletons collected.[20]

The next important discovery was made when a German expedition to the Tendaguru, then part of German East Africa, from 1909 to 1912 excavated over a thousand bones of Kentrosaurus. The finds increased the known variability of the group, Kentrosaurus being rather small and having long rows of spikes on the hip and tail.

After 1912, Western researchers for a long time failed to identify any new stegosaurs, scientific interest in dinosaurs as whole being rather limited during the middle of the twentieth century. From the 1950s onwards, the geology of China was systematically surveyed in detail and infrastructural works led to a vast increase of digging activities in that country. This resulted in a new wave of Chinese stegosaurian discoveries, starting with Chialingosaurus in 1957. Chinese finds of the 1970s and 1980s included Wuerhosaurus, Tuojiangosaurus, Chungkingosaurus, Huayangosaurus, Yingshanosaurus and Gigantspinosaurus. This increased the age range of good fossil stegosaurian material, as they represented the first relatively complete skeletons from the Middle Jurassic and the Early Cretaceous. Especially important was Huayangosaurus that provided unique information about the early evolution of the group.

Towards the end of the twentieth century the so-called Dinosaur Renaissance took place, a vast increase in scientific attention given to the Dinosauria. As part of this development, the rate of dinosaurian discoveries quickly picked up. However, this has not resulted in a peak of stegosaurian finds, partly because most new sites are from the Cretaceous, when stegosaurian diversity had declined. In 2007, Jiangjunosaurus was reported, the first Chinese dinosaur named since 1994. Nevertheless, European and North-American sites have become productive again during the 1990s, Miragaia having been found in Portugal and a number of relatively complete Hesperosaurus skeletons having been excavated in Wyoming. Apart from the fossils per se, important new insights have been gained by applying the method of cladistics, allowing for the first time to exactly calculate stegosaurian evolutionary relationships.

The following timeline shows the date of descriptions for valid stegosaurian genera beginning in 1824, when the first non-avian dinosaur, Megalosaurus, was formally described. The fossils themselves were found earlier; in the case of Loricatosaurus e.g. there is a gap of 107 years between the discovery and the naming of the genus.